1. Field of the Invention
The present invention relates to a method for producing a crankshaft having a hollow hole, and relates to a production apparatus therefor.
2. Related Art
A crankshaft of an internal-combustion engine is provided with a journal shaft. A crankpin that is parallel to the journal shaft is connected thereto via an arm. A counterweight is formed in the arm, and the forming position of the counterweight with respect to the journal shaft is at the opposite side to the connecting position of the crankpin.
In the crankshaft, from the viewpoint of improvement of fuel efficiency, weight saving by forming a hollow hole in the crankpin may be performed. When the hollow hole is formed in the crankpin, rigidity of the crankshaft is slightly affected by the forming of the hole, so that the hole is preferably formed in the crankpin. As a method for forming the hole, machining, and a method in which a crankpin is formed from a tubular material and is connected to a main body of the crankshaft, may be exemplified. In both methods, complicated steps are required, so that the production cost is higher. For reducing the production cost, forming of the main body of the crankshaft and forming the hole are effectively performed by forging. However, the crankshaft has a complicated structure as described above, so that forming the hole and the crankshaft cannot be simultaneously performed in the conventional forging.
That is, in the conventional forging, an upper die and a lower die of a die set are moved in a moving direction of a press ram so as to hold a forming object, and the material is loaded into a cavity of the closed die set, so that a required shape of the material can be obtained (for example, see Japanese Patent Application, First Publication No. 2003-343592). When a crankshaft is formed by such a forging, the crankshaft must be disposed in the die set in a condition in which the axial direction of the crankshaft is perpendicular with respect to the moving direction of the press ram, so that a punch for forming the hole cannot be inserted from the axial direction of the crankshaft into the crankpin. Therefore, in conventional forging, hot multiple processing, forming burrs, is ordinarily used. In this case, crankpins and the arms are formed in a solid condition and are provided with a draft inclination for removal from the die set so that the separatable die set composed of the upper die and the lower die can be applied thereto.
As is explained above, in forging using a die set having a closable structure, forming a hole in a crankpin is difficult, so that the hole is formed after forging and trimming in a condition in which the crankshaft is not disposed in a closed space.
Specifically, for example, when holes 1013A and 1013B are formed in a crankshaft 1010 shown in FIG. 5, as shown in FIG. 6A, one surface of one arm 1012 (the rightmost arm 1012 in FIG. 5) connecting the crankpin 1013 is abutted at a lower die 1001 (the shaded area in FIG. 6A) and a punch 1002 is inserted into the crankpin 1013 from an upper side of another arm 1012. The holes 1013A and 1013B are individually formed by insertion of the punch 1002. In forming the hole 1013B, an adjacent portion of the arm 1012 interferes with the punch 1002, so that the punch 1002 is inserted into the crankpin 1013 from an inclined direction (the direction indicated by the dashed arrowhead line in FIG. 5). In this case, a mark of abutting may remain on the portion abutting at the lower die 1001 in the one surface of the arm 1012 (the rightmost arm 1012 in FIG. 5).
However, in the method shown in FIG. 6A, as shown in FIG. 6B, the radial direction (the directions indicated by the arrows in FIG. 6B) of the crankpin 1013 is not restrained, so that dimensional accuracy of the crankpin 1013 is degraded by forming the holes 1013A and 1013B thereto. Specifically, in the crankpin 1013, shrinkages of the opening end surfaces of the holes 1013A and 1013B occur in the axial direction of the crankpin 1013 and the material thereof is extended toward the radial direction (the direction indicated by arrows in FIG. 6B). Therefore, balance of the arm 1012 must be corrected in a way such as forming plural holes (not shown) in the circumference of the counterweight of the arm 1012.
Then, using a preformed body previously formed in an applicable shape for a vertically separatable die set as a forming object, coining may be performed by a forging apparatus perpendicularly moving with respect to the moving direction of the press ram. However, improvement in dimensional accuracy in forming the hole could not be obtained by this side forming technique. This problem is a first object of the present invention.
For forming a hole in the crankpin, a forging apparatus having a side forming punch, which is moved toward the perpendicular direction with respect to the moving direction of the press ram, may be used (For example, see Japanese Patent Application, First Publication No. 2007-245229). As a driving source of the side-forming punch of the forging apparatus, a cam mechanism is ordinarily used so that the cam mechanism having a simplified structure compared to a servomotor and oil pressure means can be disposed in the inside of a die set and can linearly follow movement of the press ram.
FIG. 12 is a conceptual diagram for explanation of movement of a side-forming punch 2020 by a cam mechanism 2010. The cam mechanism 2010 is provided with a cam driver 2012 for driving cams 2011 and 2012, and a cam holder 2013 by which these members 2011 and 2012 are slidably held. A side-forming punch 2020 is provided at one side surface of the cam 2011 facing a die set and another side surface thereof on the opposite side to the die set is inclined. Furthermore, a bottom surface of the cam driver 2012 is disposed with a predetermined clearance with respect to the inclined surface of the cam 2011 in the initial condition of the cam driver 2012 and is inclined-surface slid with respect to the inclined surface of the cam 2011 in an operation of the cam 2011. In the cam mechanism 2010, when an upper plate 2031 is downwardly moved at a predetermined distance toward a lower plate 2032 by the press ram (not shown), the inclined surfaces of the cam 2011 and the cam driver 2012 contact each other. These inclined surfaces slide relative to each other by the further downward movement of the upper plate 2031, so that the side forming punch 2020 is moved toward the inside of the die set along the horizontal direction.
However, when the crankshaft is formed by this forging apparatus, the crankshaft must be disposed in the die set in a condition in which an axial direction of the crankshaft is perpendicular to the moving direction of the press ram, so that the side forming punch 2020 must be inserted into the crankpin from the axial direction of the crankshaft. Therefore, when the holes are formed in each crankpin of the crankshaft applied to a multiple-cylinder engine, the side-forming punches intersect.
Specifically, in the side-forming using the cam mechanism 2010, the cam 2011 is moved in conjunction with the movement of the press ram so that the side-forming punch 2020 provided to the cam 2011 is inserted and reaches to the deepest portion in the crankpin when the press ram reaches the bottom dead point. After this operation, the press ram is moved toward a top dead point, and the side-forming punch is pulled out from the crankpin. Therefore, for example, as shown in FIG. 13, when the hole 2044 is formed in each crankpin 2043 of the crankshaft 2040 having a full counterweight structure in a four-cylinder engine, side-forming punches 2021 and 2022 intersect each other and side-forming punches 2023 and 2024 also intersect each other (See areas framed by dashed lines in FIG. 13). Therefore, plural holes cannot be formed at the positions in which the side-forming punches intersect each other in one stroke from the top dead point to the bottom dead point of the press ram. Reference numeral 2041 indicates a journal shaft and reference numeral 2042 indicates a crank arm in FIG. 13.
For these reasons, the side-forming using the cam mechanism cannot be applied to the forging for a crankshaft having a structure in which the side-forming punches intersect each other. Therefore, this intersection may be prevented by individual control with respect to the side forming punches using a servomotor and oil pressure means that are separate from the movement of the press ram as a driving source of the side-forming punch. However, in this technique, a large space for setting a machine, such as an actuator is required, so that not only are workability and productivity degraded, but also the press equipment is larger so that a range of movement thereof is extended to the outside of the die set. This problem is a second object of the present invention.
In the conventional production of a crankshaft, when a hollow forming operation is performed, for example, a crankshaft production apparatus 3001 shown in FIG. 20 is used. The crankshaft production apparatus 3001 is provided with a transfer-type press machine 3010, a robot 3020, and a hollow forming press machine 3030. In the transfer-type press machine 3010, a main forming operation is performed with respect to a work W and the robot 2020 carries a preformed product P of the crankshaft obtained by the transfer-type press machine 3010 to the hollow forming press machine 3030, whereby the hollow forming is performed with respect to the preformed product P of the crankshaft.
In the transfer-type press machine 3010, a press ram 3012 is disposed at an upper side of a press bolster 3011 and is facing thereto. An upset die set 3021, a rough forging die set 3022, a finishing die set 3022 and a trimming die set 3024 that can be vertically divided into upper dies and lower dies are disposed between the press bolster 3011 and the press ram 3012 in order of the forming operations toward the progressing direction thereof (toward the right hand direction in FIG. 20). In these die sets, the main forming operation (upsetting, rough forging, finishing, and trimming operations) is performed with respect to the work W. The upper dies of the die sets 3021 to 3024 are moved toward the lower dies by the press ram. The lower dies of the die sets 3021 to 3024 are fixed on an upper surface of the press bolster 3011.
FIG. 21 is a top view showing a schematic structure of the crankshaft production apparatus 3001 shown in FIG. 20. Feed bars 3013 and 3014 and clamp clicks 3013A to 3013D and 3014A to 3014D shown in FIG. 21 are not shown in FIG. 20. At the front and the rear sides of the lower dies of the die sets 3021 to 3024, the feed bars 3013 and 3014 that are extended toward the forming operations progressing direction are provided. Only one punch of a hollow forming mechanism 3042 is shown in FIG. 20 for convenience.
In the first feed bar 3013, the first clamp clicks 3013A to 3013D are provided and, in the second feed bar 3014, the second clamp clicks 3014A to 3014D are provided facing the first clamp clicks 3013A to 3013D. The first clamp clicks 3013A to 3013D and the second clamp clicks 3014A to 3014D are disposed along the extending direction of the feed bars 3013 and 3014 at a predetermined interval (the interval between the die sets). The first clamp clicks 3013A to 3013D and the second clamp clicks 3014A to 3014D hold the work W and are moved by the feed bars 3013 and 3014 along the extending direction thereof. In the feed bars 3013 and 3014, the works W after the forming operations on the lower dies of each die set are held by the clamp clicks and are transferred to the next lower dies for the next forming operations (For example, see Japanese Patent Application, First Publication No. 2008-87048).
In such a transfer-type press machine 3010, as shown in FIGS. 22A to 22C, after disposing the works W1 and W2 in the die sets, operations such as die-set clamping, die-set opening, transferring the work W to the die set for the next forming operation, the die-set clamping in the next die set, and the die-set opening thereof are sequentially performed. In this case, each operation is performed by single press ram 3012, and the transfer to the next operation is performed by the feed bars 3013 and 3014.
For example, as shown in FIG. 22A, the work W1 which was subjected to finishing forming in the finishing die set 3023 is loaded into the trimming die set 3024. In the trimming die set 3024, as shown in FIG. 22B, a burr B is punched out from the work W1 and the preformed product P of the crankshaft can be obtained. Specifically, as shown in FIG. 23A, an upper die 3024A is downwardly moved toward the work W1 on a lower die 3024B, so that the burr B is punched out from the work W1 by the upper die 3024A and the burr holding die set 3024C on which the burr B of the work W1 is disposed. Thus, the preformed product P of the crankshaft can be obtained.
Next, after the preformed product P is carried to the press machine 3030 by the robot 3020, the burr B that remained on the lower die 3024B, as shown in FIG. 22C, is upwardly moved by the upward movement of the burr holding die set 3024C and is transferred to a burr conveyer 3015 in a condition in which the burr B is held by the clamp clicks 3013D and 3014D. Specifically, as shown in FIG. 23B, according to the upward movement of the upper die 3024A, the burr B is lifted by the burr holding die set 3024C of the lower die 3024B. Next, the burr B is held by the clamp clicks 3013D and 3014D and is transferred to the burr conveyer 3015 by the feed bars 3013 and 3014.
In the hollow forming press machine 3030, a hollow hole is formed in the preformed product P, so that the formed product can be obtained (the hollow forming operation). The hollow hole is formed for weight saving from the viewpoint of improvement of fuel efficiency. In this case, when the hole is formed in the crankpin, the rigidity of the crankshaft is not so decreased, so that the hole is formed in the crankpin.
In the hollow forming operation, an upper die of a die 3041 is moved downward by a press ram 3032 toward a lower die on which the preformed product P is disposed. By this operation, the preformed product P is closed in the die set 3041 and is inserted by a punch of a hollow forming mechanism 3042 from the perpendicular direction with respect to the moving direction of the press ram 3032. In such a side-forming processing, as a driving source of the punch, means such as a cam slide, an oil hydraulic cylinder, or a servomotor is used (For example, see Japanese Patent Application, First Publication No. 61-143727). In FIG. 20, reference numerals 3024L and 3041L indicate work holding cushions such as a spring, a gas cushion, or a hydraulic cushion, and reference numeral 3031 indicates a press bolster.
When the above hollow forming processing is performed, the hollow forming press machine 3030 is separately used from the transfer-type press machine 3010, so that the production cost is increased and selectivity in the layout of the machines is degraded. Therefore, unification of the transfer type press machine 3010 and the hollow forming press machine 3030 may be performed so that the main forming operation and the hollow forming operation can be sequentially performed in a single apparatus (For example, the hollow forming die set 40341 is provided at the adjacent position on the forming operations progressing direction side of the trimming die set 3024 and the feed bars 3013 and 3014 are extended toward the forming operations progressing direction, whereby the clamp clicks are provided).
However, such a unification of these machines is difficult for the following reasons. That is, in the transfer by the feed bars 3013 and 3014, a loading position of the preformed product P is determined based on a pitch (moving amount) of the feed bars 3013 and 3014 and the pitch is fixed based on an interval between the die sets. Therefore, in the transfer by the feed bars 3013 and 3014, the pitch (the moving amount) at the hollow forming part cannot be individually modified, so that accuracy in loading the preformed product P into the hollow forming die set 3041 is degraded compared to the transfer by the robot 3020 by which the loading position of the preformed product P can be modified according to the position of the die set. Furthermore, in the hollow forming die set 3041, the clearance between the cavity for forming a finally required shape and the preformed product P is very small, so that an insertion of the preformed product P into the cavity is difficult.
Furthermore, the single press ram 3012 is applied to each die set. Therefore, when adjustment of the thickness (the adjustment in the height direction) of the hole is individually performed by the modification of the shut-height in the hollow forming operation, such an adjustment in the height direction affects the other operations. As a result, selectivity of the material shape loaded in the hollow forming operation is lost. This problem is a third object of the present invention.